The present invention relates to a helically-shaped intake port of an internal combustion engine.
A helically-shaped intake port normally comprises a helical portion formed around the intake valve of an engine, and a substantially straight inlet passage portion tangentially connected to the helical portion. However, if such a helically-shaped intake port is so formed that a strong swirl motion is created in the combustion chamber of an engine when the engine is operating at a low speed under a light load, that is, when the amount of air fed into the cylinder of the engine is small, since air flowing within the helically-shaped intake port is subjected to a great flow resistance, a problem occurs in that the volumetric efficiency is reduced when the engine is operating at a high speed under a heavy load, that is, when the amount of air fed into the cylinder of the engine is large.
In order to eliminate such a problem, the inventor has proposed a helically-shaped intake port in which a bypass passage, branched off from the inlet passage portion and connected to the helix terminating portion of the helical portion, is formed in the cylinder head of an engine. A flow control valve is arranged in the bypass passage and opened when the engine is operating under a heavy load at a high speed. In this helically-shaped intake port, when the engine is operating under a heavy load at a high speed, a part of the air introduced into the inlet passage portion is fed into the helical portion of the helically-shaped intake port via the bypass passage. Consequently, since the flow area of the intake port is increased when the engine is operating under a heavy load at a high speed, it is possible to increase the volumetric efficiency. However, in this helically-shaped intake port, since the bypass passage is formed by a tubular passage which is completely separated from the inlet passage portion, the bypass passage has a relatively great flow resistance. In addition, since it is necessary to form the bypass passage at a position adjacent to the inlet passage portion, the cross-sectional area of the bypass passage is restricted by the presence of the inlet passage portion. Consequently, it is difficult to obtain a satisfactorily high volumetric efficiency. In addition, the helically-shaped intake port has a complicated construction itself and, thus, if the bypass passage completely separated from the inlet passage portion is additionally provided, the entire construction of the intake port becomes extremely complicated. Therefore, it is considerably difficult to form the helically-shaped intake port equipped with such a bypass passage in the cylinder head.